Freeze drying apparatus



Sept. 20, 1966 u. HACKENBERG FREEZE DRYING APPARATUS Filed May 29, 1964United States Patent 3,273,259 FREEZE DRYING APPARATUS UlrichHackenberg, Alter Trassweg 9, Bensberg-Refrath, Germany Filed May 29,1964, Ser. No. 371,217 Claims priority, application Germany, June 6,1963, L 45,038 Claims. (Cl. 34-92) This invention relates to a methodand apparatus for freeze drying and more particularly to a method andapparatus for continuous freeze drying applications.

Freeze drying is a well known process in which a materialto be dried isfirst frozen and then placed inside an evacuable chamber. Water vaporremoval apparatus, such as for example a refrigerated condenser, thenabsorbs sublimating water vapor produced by heating of the frozenmaterial. A predetermined low pressure of for example 10 mms. of mercuryis continuously maintained within the vacuum chamber during the dryingprocess by means of conventional vacuum pumps communicating with thechamber.

The present invention is especially suited for continuous freeze dryingapplications which utilize a central vacuum drying chamber straddled byisolatable inlet and outlet lock chambers. This arrangement allowsproduct carriers to be periodically loaded into the inlet lock chamberand removed from the outlet lock chamber while continuously maintainingthe vacuum drying process in the intermediate drying chamber.

During the freeze drying process high heat quantities per time unit mustbe conducted to the drying material for both physical and economicreasons. One method of meeting these heating requirements is to transmitheat over small distances from large heating surfaces to the dryingmaterial, utilizing the sublimating vapor from the material as theprimary heat conductor. In this arrangement the heating surfaces arenormally maintained at relatively low temperature such as, for example,60 C., but the large heating surfaces and small conduction distancespermit the transmission of large heat quantities from the heatingsurfaces to the material. Another heating method entails the use ofrelatively hot heating elements of, for example, more than 100 C. andtransmitting heat by radiation to the material over relatively longdistances. This method, however, has the disadvantage of sometimesproducing thermal damage to the drying material.

As improved heating method utilizes both above methods by transmittingheat from the heating elements to the drying material in two stages. Thefirst stage consists of radiating heat from relatively hot heatingelements of, for example, 200 C. to containers filled with the dryingmaterial which containers are designed to also shield the dryingmaterial from direct heat radiation. The heat thus absorbed by thematerial containers is then transmitted to the drying material by bothconduction and convection. The two stage heating process was disclosedin US. Patent application 295,617, filed July 15, 1963, by W. Nerge, H.Ehlers, and the applicant.

The above improved heating method while reducing the time required forthe drying process exhibits some economic disadvantages. By utilizinghot heating elements, the vapor sublimating from the drying materialbecomes overheated resulting in increased operating costs for therequired evacuation equipment. In conventional freeze dryinginstallations the evacuation equipment typically consists ofrefrigerated condensers and mechanical vacuum pumps. The condensers andvacuum pumps are connected for gas communication with the drying chamberand are normally positioned so as to provide relatively short conductionpaths between the drying material and the evacuation equipment. Sincethe refrigerated con- 3,273,259 Patented Sept. 20, 1966 denser devicesmust absorb the heat contained in the condensing vapors, thisoverheating of the vapors produces an increased load upon the condenserdevices. Similarly, the operating costs for the mechanical vacuum pumpswhose efficiency is related to volume pumping capacity is also increasedby the formation of excessively heated vapor.

The object of this invention, therefore, is to provide a freeze dryingmethod and apparatus which reduces the time required for completion ofthe drying process and also reduces the operating costs for theevacuation equipment required.

One feature of this invention is the provision of a continuous freezedrying apparatus having an elongated cen tral drying chamber straddledby inlet and outlet lock chambers and wherein all evacuation equipmentfor the central drying chamber is connected thereto in the vicinity ofthe outlet lock chamber thereby causing the sublimating vapor to passthrough the entire chamber length and to give up excessive heat beforereaching the evacuation equipment.

Another feature of this invention is the provision of a freeze dryingapparatus of the above featured type wherein the drying chamber containsindividually controlled heating elements which allow the maintenance ofdecreasing temperature level heating plates along the travel path of thedrying material.

Another feature of this invention is the provision of a freeze dryingapparatus of the above featured types having an outlet lock chamberwhich extends laterally from the intermediate drying chamber therebypermitting advantageous location of the evacuation equipment and whereinthe drying chamber includes a portion of enlarged cross section directlyadjacent the outlet lock chamber which enlarged section allows lateralmaneuvering of the material carriers into the outlet lock chamber.

Another feature of this invention is the provision of a freeze dryingmethod wherein a material to be dried is conveyed through an elongatedevacuated drying chamber, heat is applied to the material duringprogress through the evacuated chamber, and the resulting sublimatingvapor coming from the material is removed along a path extendingsubstantially the entire length of the elongated drying chamber therebyallowing excessive heat contained in the vapor to be removed by contactwith the material moving through the drying chamber.

Another feature of this invention is the provision of a freeze dryingmethod of the above featured type wherein the heat applied to the frozenmaterial is applied at reducing temperature levels as the materialprogresses through the evacuated drying chamber.

These and other features of the instant invention will become moreapparent upon a perusal of the following specification taken inconjunction with the accompanying drawing which is a schematic showingpartly in cross section of a preferred embodiment of the invention.

Referring now to the drawing there is shown a freeze drying installationhaving a central drying chamber 11 formed by the elongated cylindricaltank 12 of circular cross section and by the housing 13 which is oflarger cross section than the elongated tank 12. Along the entire lengthof and adjacent to the inner Walls of the elongated tank 12 and enlargedhousing 13 are a plurality of separate heating plates 14 which areadapted to be independently controlled by suitable heat sources (notshown). An inlet lock chamber 15 having a hinged door 16 is connected tothe elongated tank 12 by the gate valve assembly 17. Connected for gascommunication with the in terior of the inlet lock chamber 15 throughthe valve 18 is the cold condenser 19 and the mechanical vacuum pump 21.A compressor 22 is connected to supply a refrigerant to the coldcondenser 19. The vent valve 23 3, is positioned between the outeratmosphere and the interior of the inlet lock chamber 15.

The end of the drying chamber 11 adjacent the enlarged housing 13 isbifurcated into a pair of condenser housings 25. The condenser housings25 are connected to the drying chamber 11 by isolating gate valveassemblies 26 and enclose the cold condenser elements 27. Communicatingwith each of the condenser housings 25 through gas valves 28 is themechanical vacuum pump 29. A pair of compressors 31 is connected tosupply refrigerant to each of the cold condenser elements 27.

Extending laterally from the enlarged housing 13 is the outlet lockchamber 32 which is connected thereto by the isolating gate valveassembly 33. The outlet lock chamber 32 is provided with a hinged door34 and an atmospheric vent valve 35. Connected for gas communicationwith the outlet chamber 32 through the gas valve 36 is the coldcondenser 37 and the mechanical vacuum pump 38. A compressor 39 isconnected to supply refrigerant to the cold condenser 37.

In the operation of the preferred freeze drying installation shown inthe drawing, a suitable product carrier (not shown) is first loaded witha frozen material to be dried and placed inside inlet lock chamber 15through open door 16 with the isolating gate valve 17 in a closedposition. The hinged door 16 is closed and the inlet lock chamber 15evacuated by the mechanical vacuum pump 21 through open gas valve 18.The isolating gate valve 17 is then opened and the loaded productcarrier is automatically transported into the already evacuated dryingchamber 11 on the conveyor rail 41 by a suitable motive means (notshown). The isolating gate valve 17 is again closed and the vent valve23 opened to produce atmospheric conditions in the inlet lock chamber15. Upon opening of the hinged door 16, another loaded product carrieris moved into the inlet lock chamber 15. After evacuation of the inletlock chamber 15, the second loaded product carrier is also transportedinto the drying chamber 11 through the opened isolating valve 17. Thisprocedure is continued until the entire drying chamber 11 is filled withproduct carriers filled with the material to be dried.

Inside the drying chamber 11 the heating elements 14 apply heat to thefrozen material causing sublimation of the moisture therein. Thissublimating vapor is pumped through the length of the drying chamber 11and open isolating gate valves 26 into the condenser housings 25. Thecondensable portion of this vapor is condensed on the cold surfaces ofthe cold condensers 27 while the noncondensable portion is pumped out ofthe housings through open valves 28 by the mechanical vacuum pump 29.The use of the two independent condenser housings is advantageous as itallows one of the units to be isolated from the drying chamber forcleaning and de-icing while the drying process continues with the otherunit.

As the material is gradually transported through the drying chamber 11,a dry outer layer is formed on the material due to the removal ofsublimating water vapor. This dry layer is very susceptible to thermaldamage and becomes increasingly thicker as the drying process continues.It is therefore desirable to reduce the level of applied heat as thethickness of the dry layer becomes greater. The reduced heat level canbe correlated to the material drying speed (in mms. of water removed perminute) which drying speed decreases as the thickness of the driedmaterial layer increases. For this reason the heating units 14 arecontrolled so that each successive heating unit, going in direction fromthe inlet lock chamber 15 toward outlet lock chamber 32, operates at areduced temperature. For example only, with seven independent heatingunits the preferred operating temperature for each successive unit wouldbe 280 C., 220 C., 170 0, 130 C., 100 C., 80 C., and 70 C.

The advantage of the present invention is that vapor produced in theinitial portions of the drying chamber 11 which vapor is overheated bythe higher temperature heating units is forced to travel the entirelength of the drying chamber 11 before reaching the cold condensers 27and the vacuum pump 29. While passing through the chamber 11 a portionof the heat in this vapor will be absorbed by the freeze dry componentswithin the advanced parts of the tank 12 and housing 13 and especiallyby the frozen material contained in these parts. The heat absorption bythe frozen material is particularly beneficial since it assists inproviding the desired sublimation of the frozen moisture. The heat lostin this way greatly reduces the average temperature of the vaporreaching the cold condensers 27 and the mechanical vacuum pump 29. Thisreduced temperature in turn reduces both the energy requirements formaintaining the cold condensers 27 at their desired operatingtemperature of, for example, 40 C. and the pumping capacity requirementsfor the mechanical vacuum pump 29.

As each product carrier reaches the enlarged housing 13, the materialcarried has been completely dried and the carrier is transported intothe outlet lock chamber 32 through open isolating gate valve 33. Theisolating valve 33 is then closed and the vent valve 35 opened toproduce atmospheric conditions within the outlet chamber 32. Afterremoving the dried material, the hinged door 34 and the vent valve 35are closed and the outlet lock chamber 32 is again evacuated by themechanical vacuum pump 38 through the open gas valve 36. The isolatingvalve 33 is then opened preparing the outlet chamber to receive anotherproduct carrier containing dried material.

The lateral positioning of the outlet lock chamber 32 permits theadvantageous arrangement of the bifurcated condenser housings 25.However, the use of the lateral outlet chamber is made possible by theenlarged housing 13. This enlarged housing permits lateral maneuveringof the product carriers which in the interest of space economy normallyconform very closely to the inner wall confines of the cylindrical tank12.

Thus the invention discloses a freeze drying installation which providesefiicient, safe heating of the drying material and also greatly reducesthe operating costs of the evacuation equipment required.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. For example only, the materialto be dried could be directly frozen within the inlet lock chamber 15 byevaporative cooling produced by the cold condenser 19. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise and as specifically described.

What is claimed is:

1. A freeze drying apparatus comprising;

(a) an elongated evacuable drying chamber having inlet, outlet andintermediate portions,

conveyor means adapted to transport a material to be dried through saiddrying chamber, heating means positioned within said drying chamber andadapted to heat the material to be dried; (b) an avacuable inlet lockchamber connected to said drying chamber inlet portion,

inlet lock valve means for isolating said inlet lock chamber from saiddrying chamber, an inlet door means adapted to provide access into saidinlet lock chamber, inlet lock evacuation means adapted to provideevacation of said inlet lock chamber; (c) an evacuable outlet lockchamber connected to said drying machine outlet portion,

outlet lock valve means for isolating said outlet lock chamber from saiddrying chamber, an outlet door means adapted to provide access into saidoutlet lock chamber, outlet lock evacuation means adapted to provideevacuation of said outlet lock chamber; and

(d) drying chamber evacuation means directly connected to and adapted toprovide evacuation of the entire said drying chamber,

said drying chamber evacuation means being connected to the outlet endof said drying chamber thereby providing a gas evacuation path along theentire length of said drying chamber and wherein no additionalevacuation devices are directly connected to either said drying chamberinlet or intermediate portions.

2.'An apparatus according to claim 1 wherein said drying chamberevacuation means includes refrigerated condenser and mechanical vacuumpump means.

3. An apparatus according to claim 2 wherein said drying chamberevacuation means includes at least two refrigerated condenser pumpsdirectly connected to said drying chamber outlet portion and includingcondenser valve means for independently isolating each of saidrefrigerated condenser pumps from said drying chamber.

4. An apparatus according to claim 3 wherein said elongated dryingchamber is formed by an elongated cylindrical tank portion of uniformcross section and an end portion of enlarged cross section, and saidoutlet lock chamber extends laterally from said end portion of enlargedcross section.

5. An apparatus according to claim 1 wherein said elongated dryingchamber is formed by an elongated cylindrical tank portion of uniformcross section and an end portion of enlarged cross section, and saidoutlet lock chamber extends laterally from said end portion of enlargedcross section.

'6. An apparatus according to claim 1 wherein said heating means withinsaid drying chamber comprises a plurality of individually controlledunits thereby permitting the application of reducing heatingtemperatures to the material being dried as the material is transportedthrough said drying chamber.

7. An apparatus according to claim 6 wherein said drying chamberevacuation means includes refrigerated condenser and mechanical vacuumpump means.

8. An apparatus according to claim 7 wherein said drying chamberevacuation means includes at least two refrigerated condenser pumpsdirectly connected to said drying chamber outlet portion and includingcondenser valve means for independently isolating each of saidrefrigerated condenser pumps from said drying chamber.

9. An apparatus according to claim 8 wherein said elongated dryingchamber is formed by an elongated cylindrical tank portion of uniformcross section and an end portion of enlarged cross section, and saidoutlet lock chamber extends laterally from said end portion of enlargedcross section.

10. An apparatus according to claim 6 wherein said elongated dryingchamber is formed by an elongated cylindrical tank portion of uniformcross section and an end portion of enlarged cross section, and saidoutlet lock chamber extends laterally from said end portion of enlargedcross section.

References Cited by the Examiner UNITED STATES PATENTS 888,257 5/1908Passbury 3492 2,523,552 8/1950 Birdseye 345 2,528,476 10/1950 Roos 3452,751,687 6/1956 Colton 34-92 2,858,795 11/1958 Walker 3492 3,192,6457/1965 Oetjen 3492 WILLIAM J. WYE, Primary Examiner.

1. A FREEZE DRYING APPARATUS COMPRISING; (A) AN ELONGATED EVACUABLEDRYING CHAMBER HAVING INLET, OUTLET AND INTERMEDIATE PORTIONS, CONVEYORMEANS ADAPTED TO TRANSPORT A MATERIAL TO BE DRIED THROUGH SAID DRYINGCHAMBER, HEATING MEANS POSITIONED WITHIN SAID DRYING CHAMBER, BER ANDADAPTED TO HEAT THE MATERIAL TO BE DRIED; (B) AN AVACUABLE INLET LOCKCHAMBER CONNECTED TO SAID DRYING CHAMBER INLET PORTION, INLET LOCK VALVEMEANS FOR ISOLATING SAID INLET LOCK CHAMBER FROM SAID DRYING CHAMBER, ANINLET DOOR MEANS ADAPTED TO PROVIDE ACCESS INTO SAID INLET LOCK CHAMBER,INLET LOCK EVACUATION MEANS ADAPTED TO PROVIDE EVACATION OF SAID INLETLOCK CHAMBER; (C) AN EVACUABLE OUTLET LOCK CHAMBER CONNECTED TO SAIDDRYING MACHINE OUTLET PORTION, OUTLET LOCK VALVE MEANS FOR ISOLATINGSAID OUTLET LOCK CHAMBER FROM SAID DRYING CHAMBER, AN OUTLET DOOR MEANSADAPTED TO PROVIDE ACCESS INTO SAID OUTLET LOCK CHAMBER, OUTLET LOCKEVACUATION MEANS ADAPTED TO PROVIDE EVACUATION OF SAID OUTLET LOCKCHAMBER; AND (D) DRYING CHAMBER EVACUATION MEANS DIRECTLY CONNECTED TOAND ADAPTED TO PROVIDE EVACUATION OF THE ENTIRE SAID DRYING CHAMBER,SAID DRYING CHAMBER EVACUATION MEANS BEING CONNECTED TO THE OUTLET ENDOF SAID DRYING CHAMBER THEREBY PROVIDING A GAS EVACUATION PATH ALONG THEENTIRE LENGTH OF SAID DRYING CHAMBER AND WHEREIN NO ADDITIONALEVACUATION DEVICES ARE DIRECTLY CONNECTED TO EITHER SAID DRYING CHAMBERINLET OF INTERMEDIATE PORTIONS.